As semiconductor devices are scaled to smaller dimensions, generally in the sub-0.1 .mu.m region, it is highly desirable and generally necessary to fabricate such devices with smaller junction depths and a controllable pocket implant adjacent to the shallow junctions to reduce short-channel effects (i.e., reduced threshold voltage rolloff) and reduced in gate length. The pocket implant is a doped implanted region which is oppositely doped to the junction regions. A problem that arises with such small geometries is that, with very short channel lengths, the implant profile cannot be adequately controlled and shallow junctions and/or well controlled thicknesses of doped layers generally cannot be formed by simple implantation.
An example of such a prior art device is shown in FIGS. 1a and 1b wherein there is shown a semiconductor substrate 1, for example doped p-type, having a gate electrode 3 spaced from the substrate by a dielectric layer 5. Shallow doped extension regions 7 (denoted herein as drain extension regions and, for example, doped n-type) may be formed on each side of the gate electrode 3 with or without sidewall dielectric spacers 2 provided adjacent to the gate electrode 3 prior to formation of the doped extension regions 7. Pocket regions 9 of doping type opposite (e.g., p-type) to that of the drain extension regions 7 may be formed by means of implantation prior to or after formation of the drain extension regions 7. Typically, the pocket region 9 extends beyond the drain extension regions 7 in both the lateral and vertical directions. whereby a large bottomwall capacitance can result due to the n/p junction region formed at the bottom of the drain extension region due to the overlap with the pocket region 9 over the entire active area. The doping in the pocket region 9 from the pocket process may be of higher concentration than the doping of the substrate 1.
To reduce this bottomwall capacitance over the entire active area, a deeper source/drain region 10 (in this example, n-type) can be formed after formation of sidewall spacers 12 so that the bottomwall overlap of the deeper source/drain region 10 and the pocket regions 9 is eliminated, thus reducing the bottomwall capacitance in these regions as shown in FIG. 1b.
When dealing with sub-0.1 .mu.m geometries, the gate width dimensions are in the 200 to 900 angstrom region, thereby leaving a channel region on the order of about 100-800 angstroms. Implants cannot be adequately controlled in accordance with the prior art semiconductor fabrication techniques when such small dimensions are involved.